tri mutate: load survivors.t27 and lab.t27 through gen-rust, drop the hand copies #7472

Problem

cli/tri/src/mutate.rs holds two hand-written Rust copies of rules that already live in t27:

• the survivor gate (gate_on, unaccepted, survivor_exit, mutate_exit, near line 1325), a copy of specs/tri/mutate/survivors.t27;
• mod lab (133 lines, near line 1376), a copy of specs/tri/mutate/lab.t27.

Each copy is kept honest by a test that evaluates every assert row of its spec against the Rust (the_gate_agrees_with_every_assert_row_of_its_spec, the_lab_rules_agree_with_every_assert_row_of_their_spec). That is two homes for one rule, plus a small evaluator to compare them. Since #7433, mutate.rs loads specs/tri/mutate/census.t27 through t27c gen-rust (gen/rust/tri/mutate/census.rs, a #[path] module, as cli/t27b loads check_budget.rs), so the copies are no longer needed.

Plan

• t27c gen-rust specs/tri/mutate/survivors.t27 and specs/tri/mutate/lab.t27 into gen/rust/tri/mutate/ (force-added; L2 compares them with the spec).
• Replace each hand copy with a #[path] module of the generated file; call sites use the module's names.
• Drop the agreement tests and their evaluator once nothing is left to compare; the spec's own tests compile into the generated module.
• A rule the generator cannot lower yet is filed against t27c and its copy stays, named in the PR.

Acceptance criteria

• grep -c 'fn survivor_exit\|^mod lab {' cli/tri/src/mutate.rs prints 0.
• cargo test --release -p tri mutate:: passes on the lab, and tri mutate spec --file specs/queen/priority.t27 --accepted F gives the same exit and the same lines as before the change.
• git diff --numstat shows more hand-written Rust lines removed than added.

Boundary

cli/tri/src/mutate.rs
gen/rust/tri/mutate/

Refs #7433, #7303, #7050

User Scenarios

• Given cli/tri/src/mutate.rs contains hand-written copies of survivors.t27 and lab.t27 rules, when t27c gen-rust generates Rust modules for both specs into gen/rust/tri/mutate/, then the hand-written copies are replaced with #[path] imports of the generated modules and call sites use the module's names.
• Given agreement tests (the_gate_agrees_with_every_assert_row_of_its_spec, the_lab_rules_agree_with_every_assert_row_of_their_spec) and their evaluator exist to compare hand-written Rust against spec asserts, when the hand-written copies are removed, then the agreement tests and evaluator are dropped because the spec's own tests compile into the generated modules.

Requirements

• FR-001: t27c gen-rust MUST generate gen/rust/tri/mutate/survivors.rs from specs/tri/mutate/survivors.t27 and gen/rust/tri/mutate/lab.rs from specs/tri/mutate/lab.t27.
• FR-002: The hand-written survivor gate (gate_on, unaccepted, survivor_exit, mutate_exit near line 1325) in cli/tri/src/mutate.rs MUST be replaced with a #[path] module importing the generated survivors.rs.
• FR-003: The hand-written mod lab (133 lines near line 1376) in cli/tri/src/mutate.rs MUST be replaced with a #[path] module importing the generated lab.rs.
• FR-004: The agreement tests and their evaluator code MUST be removed once no hand-written copies remain to compare.
• FR-005: cargo test --release -p tri mutate:: MUST pass and tri mutate spec --file specs/queen/priority.t27 --accepted F MUST produce the same exit code and output lines as before the change.